Generally, there has been often provided so-called an axial flow steam turbine, having large capacity, including a plurality of sages, arranged along steam flow direction, each comprising in combination a turbine nozzle (turbine stationary (stator) blade) and a turbine moving or movable (rotor) blade.
The axial flow steam turbines will be roughly classified into reaction type and impulse type.
The steam turbine of the impulse type causes thermal energy of a steam to perform more expansion work using each turbine nozzle, transforms the steam after the expansion work to a deflected flow using each turbine moving blade, and guides the resultant deflected flow to the next stage.
In the turbine nozzle that converts most of the thermal energy of the steam to kinetic energy, a large pressure difference occurs between a steam inlet and a steam outlet of the turbine nozzle. To deal with this pressure difference, therefore, the turbine nozzle adopts a diaphragm structure as shown in FIG. 24.
The turbine nozzle of the diaphragm structure shown in FIG. 24 is constituted as follows. A ring body 1 is divided into two portions on a horizontal joint surface 2, both ends of nozzle blades (nozzle plates) 3 arranged in ring columns are supported by a diaphragm outer ring 4 and a diaphragm inner ring 5, and a labyrinth packing mounting groove 6 is provided in an inner periphery of the diaphragm inner ring 5 that faces a turbine shaft (not shown).
Further, the turbine nozzle is so-called a weld-type turbine nozzle in which at a time when the nozzle blade 3 is connected to the diaphragm outer ring 4 and the diaphragm inner ring 5, the nozzle blade 3 is fixedly attached thereto by welding portions 8a and 8b through wear plates 7a and 7b, respectively, as shown in FIG. 25.
On the other hand, in so-called a counter-flow (double flow) turbine that divides the steam flow to a left flow and a right flow at its inlet as shown in FIG. 30, at a time when top sides of a first divided-flow nozzle blade 49 and a second divided-flow nozzle blade 50 are supported by a first divided-flow diaphragm outer ring 52 and a second divided-flow diaphragm outer ring 53, respectively, the first and second divided-flow nozzle blades 49 and 50 are fixedly attached to the first and second divided-flow diaphragm outer rings 52 and 53 by welding portions 54a and 54b and bottoms of the first and second divided-flow nozzle blades 49 and 50 are fixed by welding portions 54c and 54d using a shared diaphragm inner ring 51 shared between the first and second divided-flow nozzle blades 49 and 50, respectively.
The weld-type turbine nozzles as shown in FIG. 25 have been employed long and have given actual results. However, as international competition has been increasingly harsh, the market has demanded mare strictly improved performances and cost reduction for turbine nozzles. In light of such demand, the following matters, which have not been regarded seriously, constitute important matters or problems to be considered or solved.
(1) As to performance: deterioration of performance caused by manufacturing error resulting from welding distortion in the case of the weld-type turbine nozzle.
The most serious effect of the welding distortion is the deviation of inside and outside diameters of a steam path from designed diameters, respectively. For example, as shown in FIG. 26, even if the turbine nozzle is designed into so-called a lap (step)-free state in which both a blade root portion (blade base portion) 10 and a blade tip portion (top portion) 11 are formed linearly, both the blade root portion 10 and blade tip portion 11 actually have positive (+) or negative (−) laps relative to the designed values as their respective reference positions as shown in FIG. 27 by the effect of the welding distortion.
A turbine stage efficiency has been confirmed by an experiment based on the positive or negative laps, it has been found that as the positive or negative laps are greater, the deterioration of the turbine stage efficiency is higher. For this reason, even if a method for minimizing the welding distortion is discovered by trial and error, this method naturally has its limit, and as a result of the long-time use of the turbine nozzle, great positive or negative laps often appear again.
Furthermore, a concept of so-called offset design, in which a designed position of the non-dimensional lap is set at a positive position indicated by an arrow AR at the time of design on the assumption that a negative lap occurs, has been introduced so as to try to maintain the turbine stage efficiency at the maximum value (Mmax) during the operation of the turbine nozzle. However, this method naturally has its limit, as well.
(2) As to cost: since there are many welding steps, it is difficult to realize cost reduction.
FIG. 29 illustrates one example in which manufacturing cost composition ratios of the weld-type turbine nozzle in the form of a circular graph. In the example of FIG. 29, a welding cost reaches about 38 percents of a total manufacturing cost. As a result, even if it is attempted to effectively reduce a material cost and a working cost, there is a limit to the cost reduction. In addition, since it is difficult to mechanize and automate welding operation 100 percents, it is difficult to reduce the welding cost itself, accordingly.
The present invention has been achieved under these circumstances. It is an object of the present invention to modify and thereby simplify a turbine nozzle structure and to provide a assembled nozzle diaphragm which can be easily assembled without performing a welding operation and a method of assembling such nozzle diaphragm.